The invention relates to a metal/air cell having an anode comprising zinc and an air cathode. The invention relates to a process of heat treating a copper surface forming the inside surface of the cell""s anode casing, such as by passing a heated gas in contact therewith.
Zinc/air cells are typically in the form of a miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each can have a closed end an open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The cathode casing contains an air diffuser (air filter) which lines the inside surface of the cathode casing""s closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing.
The cathode casing can typically be of nickel plated stainless steel, for example, with the nickel plate forming the cathode casing""s outside surface and stainless steel forming the casing""s inside surface. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing""s outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing""s outside surface and the copper layer forms the anode casing""s inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell""s negative terminal at the closed end of the anode casing. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell""s negative terminal and a portion of the closed end of the cathode casing can function as the cell""s positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to reduce the hydrogen gassing which can occur as a side reaction in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The gassing, if excessive, can reduce the cell capacity and increase the chance of electrolyte leakage. Such leakage can damage or destroy the hearing aid or other electronic component being powered. Many regions around the world now greatly restrict the use of mercury in electrochemical cells because of environmental concerns. If other potential gassing inhibitors are added to the anode mix instead of mercury, they either significantly reduce the anode conductivity or have to be added in quantity, thereby significantly reducing the cell""s capacity (mAmp-hrs).
U.S. Pat. No. 3,897,265 discloses a representative zinc/air button cell construction with an anode casing inserted into the cathode casing. There is disclosed an insulator between the anode and cathode casings. The anode comprises zinc amalgamated with mercury. The cell includes an assembly comprising an air diffuser, cathode catalyst, and separator at the closed end of the cathode casing facing air holes in the surface of the cathode casing.
U.S. Pat. No. 5,279,905 discloses a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. Instead, the inner layer of the anode casing has been coated with a layer of indium. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The copper layer is at least 1 microinch (25.4xc3x9710xe2x88x926 mm). The reference discloses coating the copper layer on the anode casing""s inside surface with a layer of indium. The indium layer is disclosed as being between about 1 microinch and 5 microinches (25.4xc3x9710xe2x88x926 mm and 127xc3x9710xe2x88x926 mm).
It is thus desired to produce a zinc/air cell without added mercury.
It is desired to eliminate the need to add mercury to the zinc/air cell without increasing gassing within the cell, yet while obtaining good cell performance.
An aspect of the invention is directed to heat treating the anode casing of a metal/air depolarized cell, desirably the anode casing of a zinc/air cell. The invention can be specifically directed to heat treating the anode casing of a miniature zinc/air button cell useful as a power source for hearing aids. The process of the invention is directed to heat treating the anode casing, preferably after it is stamped into shape but before anode active material is inserted therein. Such heat treatment of the anode casing after the anode casing is formed but before active material is inserted therein can be referenced herein as post heat treatment of the anode casing. This is not to say that the metal sheeting from which the anode casing is formed may be heat treated before stamping. The anode casing has a copper layer lining its inside surface. The anode casing is preferably of a triclad material comprising stainless steel having an outside layer of nickel and an inside layer of copper.
A principal aspect of the invention involves (a) heat treating said anode casing such as with a gas passed in contact therewith at a temperature between about 200xc2x0 C. and 700xc2x0 C., preferably a temperature between about 300xc2x0 C. and 600xc2x0 C., to form a heat treated anode casing, and (b) cooling the heat treated anode casing to ambient temperature. The heat treated anode casing is then preferably stored away from atmospheric air, for example, in a vacuum sealed bag, until it is ready to be filled with anode active material during cell assembly. Preferably, the anode casing is heat treated in a quartz furnace by passing a treating gas therethrough in contact with the casing. The anode casing can be placed in a quartz tube within the furnace and the treating gas passed through the furnace so that it contacts the anode casing. The treating gas is preferably a reducing gas, for example, a gas comprising hydrogen. A preferred reducing gas comprises about 5 wt. % hydrogen and 95 wt. % inert or substantially inert gas such as argon or nitrogen.
The heat treating of the anode casing can desirably be carried out in essentially two steps: a.1) an initial heating period (ramp period) wherein the furnace temperature and consequently the temperature of the treating gas passing therethrough in contact with the anode casing is gradually increased from an initial temperature to a desired elevated (soak) temperature, and a.2) a primary heating period wherein the treating gas temperature in contact with the anode casing is maintained at said elevated (soak) temperature for a set period time (soak period). The initial temperature of the gas in contact with the anode casing can be at about room temperature (20xc2x0 C.) or lower, but also can be higher, for example, a temperature between about 20xc2x0 C. and 30xc2x0 C. The soak temperature is desirably a temperature between about 300xc2x0 C. and 700xc2x0 C., preferably a temperature between about 300xc2x0 C. and 600xc2x0 C., more preferably a temperature between about 400xc2x0 C. and 600xc2x0 C. The soak temperature is preferably maintained at a constant temperature within the above stated soak temperature range, but can also be a variable temperature within the above stated soak temperature ranges. The treating gas flow in contact with the anode casing is maintained throughout the primary heating and soak period. Once the soak temperature is reached, the anode casing is desirably exposed to the flowing treatment gas at soak temperature for a relatively short period of between about 5 and 25 minutes, preferably for about 15 minutes. The treating gas can be passed through the furnace and in contact with anode casing with the gas in either laminar or turbulent flow. Thus, the rate of gas flow is not critical and a slow rate of gas flow of between about 8 and 10 cubic centimeters per minute has been determined to be satisfactory. After the soak period, the furnace is shut off and while the heat treated anode casing is still in the furnace, the casing is subjected to a cooling step wherein it is allowed to cool gradually to ambient temperature, for example, to room temperature between about 20xc2x0 C. and 30xc2x0 C. Upon cooling to room temperature the heat treated anode casing is preferably stored away from atmospheric air, for example, in a vacuum sealed bag until it is desired to fill it with anode active material during cell construction.
It has been determined that the heat treating process of the invention as applied to said anode casing reduces gassing in a zinc/air cell utilizing the heat treated casing. The resulting reduction in gassing is significant enough that the need to add mercury to the anode mixture is entirely eliminated.
It is not known with certainty why the heat treatment process of the invention causes a significant reduction in cell gassing. It is theorized that the anode casing inside layer of copper develops surface deposits of copper oxide (CuO) as well as deposits of other oxides and contaminants during the storage period which can be a period of weeks or months from the time that the casing is formed, e.g. by stamping, until it is actually used in cell assembly. Such oxides can act as a catalyst which increases the rate of the primary gassing reaction between zinc and water to produce zinc oxide and hydrogen gas. The heat treatment process of the invention is believed to reduce the amount of copper oxides and other oxides and contaminants from the surface of the copper layer thereby reducing the rate of hydrogen gas production. The heat treatment process of the invention also produces a smoother copper surface on the inside of the anode casing, which in turn reduces the number of active sites for the gassing reaction to occur.